It is well known that internal combustion engines use valves, both intake and exhaust valves, to control the admittance of the air/fuel mixture to the cylinders. Typically, the opening and closing pattern of these valves is governed by cam lobes rotating on the engine camshaft. Each cam has a base circle and a lobe and a mechanical linkage links the cam to a valve. Whilst the linkage follows the base circle, the valve remains stationary but when that linkage follows the lobe portion of the cam it is caused to push the valve open. Typically, as the linkage moves from the cam lobe back to the base circle, the valve closes under spring action.
It is known that a single cam can have two cam lobe profiles to give different valve opening/closing events. Variable valve actuation is well known and allows the mechanical linkage to transfer a portion of the total movement to the valve that would otherwise all be transferred. In this way the engine valves can be made to open and close with different timings depending on the operation required from the engine.
One such operation is engine braking. Rather than following the typical combustion cycle, an internal combustion engine can be used as a brake if it is simply allowed to compress the air in its cylinders rather than burning fuel. Once the air in a cylinder has been compressed, the energy put into compressing that air must be released and this is typically accomplished by opening an engine exhaust valve close to top dead centre of the compression stroke. However, forces generated on the engine components during engine compression braking can be higher than during normal operation. During normal engine operation, the exhaust valve is normally opened when there is minimum pressure in the engine cylinder i.e. the piston is at or near bottom dead centre about to move upwards towards the cylinder head for the exhaust stroke. During an engine compression braking event however, the exhaust valve is opened when the contents of the cylinder are compressed and therefore under high pressure. Thus to open the exhaust valve in this situation requires that the cams and linkages driving the valve not only overcome the normal biasing force of the valve return spring but also the opposing pressure in the cylinder which acts to keep the valve shut.
Thus a need exists for an improved mechanical valve control device that allows the use of one or more cam profiles per cam but which is robust and simple and thus less likely to suffer failure in the harsh environment of the typical vehicle engine.